1. Field of the Invention
The present invention relates to a Doherty power amplifier operating at 3 GHz or more, and more particularly, to a three-stage GaN HEMT (High Electron Mobility Transistor) Doherty power amplifier for high frequency applications, which is capable of implementing high efficiency in a wide range using a GaN HEMT.
2. Description of the Related Art
As well known to those skilled in the art, the efficiency of a power amplifier used in a base station or repeater decides the efficiency of the entire system. Therefore, it is important to raise the efficiency of the power amplifier. As one of methods for raising the efficiency of the power amplifier, a Doherty power amplifier may be applied. The Doherty power amplifier has high efficiency at a 6 dB back-off power (BOP) level from the saturation output power, unlike the other methods for raising the efficiency.
A WCMDA (Wideband Code Division Multiple Access) signal or WiMAX (Worldwide Interoperability for Microwave Access) signal, which has been recently used frequently, has a large peak-to-average power ratio (PAPR). Therefore, in order to secure linearity, the WCMDA signal and the WiMAX signal are used at an output power corresponding to a BOP level to which the PAPR is reflected. Therefore, since the Doherty power amplifier has high efficiency in a wide output range, the Doherty power amplifier may be applied to a base station or repeater. In particular, much research has been recently conducted on an N-stage Doherty power amplifier having high efficiency at the 6 dB BOP level or more.
When the Doherty power amplifier is designed, Si LDMOSFETs which are inexpensive and exhibit stable performance are usually used as active elements of the power amplifier. However, when a power amplifier operating at 3 GHz is designed, Si LDMOSFET cannot be used because it has low saturation velocity. However, a GaN MEMT exhibits high efficiency even at 3 GHz, because it has high saturation velocity.
FIG. 1 is a circuit diagram of a conventional three-stage Doherty power amplifier.
Referring to FIG. 1, an input signal is divided into inputs of a carrier amplifier 104, a first peaking amplifier 105, and a second peaking amplifier 106 through a three-wavy power divider 101. An input matching circuit 103 and an output matching circuit 107 optimize gains and efficiencies of the respective power amplifiers. Depending on the magnitude of the input signal, impedance of the carrier amplifier 104 is increased by λ/4 transmission lines 108 and 109. Thus, the carrier amplifier 104 may exhibit high efficiency even at a low input. In order to compensate for phase differences between the carrier amplifier 104 and the first and second peaking amplifiers 105 and 106 caused by the λ/4 transmission lines 108 and 109, a λ/4 transmission line 102 and a λ/2 transmission line 102 are inserted into the inputs of the first peaking amplifier 105 and the second peaking amplifier 106, respectively. Since the three-stage Doherty power amplifier uses the three-way power divider at the input thereof, the gain of the carrier amplifier decreases.
FIG. 2 is graph showing drain currents of the carrier amplifier 104, the first peaking amplifier 105, and the second peaking amplifier 106 in the conventional three-stage Doherty power amplifier of FIG. 1.
Referring to FIG. 2, the drain current of the carrier amplifier 104 increases in proportional to the input magnitude, and the carrier amplifier 104 is then saturated to maintain a constant current, when the second peaking amplifier 105 starts to operate. The first and second peaking amplifiers 105 and 106 start to operate at different time points, depending on the capacities of elements used in the carrier amplifier 104 and the first and second peaking amplifiers 105 and 106. The three-stage Doherty power amplifier has high efficiency at the time points where the first and second peaking amplifiers 105 and 106 respectively operate. When a GaN HEMT is used as the carrier amplifier 104, the following problem may occur: when high input power is applied after the GaN HEMT is saturated, a current may flow in the gate of the GaN HEMT, thereby causing a breakdown.
The above-described conventional three-stage power amplifier exhibits high efficiency at a large BOP level. A power amplifier operating at a frequency of 3 GHz should use a GaN HEMT. However, when the magnitude of an input signal increases after output power of the carrier amplifier is saturated, a current of dozens mA or more flows in the gate of the carrier amplifier such that the amplifier cannot endure a large input magnitude. Then, it is impossible to design the three-stage Doherty power amplifier.